The invention relates to developing charge patterns.
The formation and development of images on an imaging surface which may be the surface of a photoconductor, by electrostatic means is well known. The basic xerographic process, as disclosed by C. F. Carlson in U.S. Pat. No. 2,297,691 involves placing a uniform electrostatic charge on a phonoconductive insulating layer, exposing the layer to a light-and-shadow image to dissipate the charge on the areas of the layer exposed to the light, and developing the resulting charge pattern by depositing on the image a finely-divided marking material referred to in the art as "toner". The toner will normally be attracted to those areas of the layer which retain a charge, thereby forming a toner image corresponding to the charge image. This powder image may then be transferred to a support surface such as paper. The transferred image may subsequently be permanently affixed to a support surface as by heat. Instead of latent image formation by uniformly charging a photoconductive layer and then exposing the layer to a light-and-shadow image, one may form the charge pattern by directly charging an image surface in image configuration. The powder image may be fixed to the imaging surface if elimination of the powder image transfer step is desired. Other suitable means such as solvent or overcoating treatment may be substituted for the foregoing heat fixing steps.
Several methods are known for applying a developer to a charge pattern to be developed. One development method as disclosed by E. N. Wise in U.S. Pat. No. 2,618,552 is known as "cascade" development. Another method of developing charge patterns is the "magnetic brush" process as disclosed for example, in U.S. Pat. No. 2,874,063. Still another development technique is the "powder cloud" process as disclosed by C. F. Carlson in U.S. Pat. No. 2,221,776.
An additional dry development system involves developing a charge pattern with a powdered developer material, the powder having been uniformly applied to the surface of a powder applicator. The charge pattern is brought close enough to the developer powder applicator so that the developer powder is pulled from the powder applicator to the charge bearing surface in image configuration. The charge pattern and powder applicator may desirably be brought in contact including contact under pressure to affect development. The powder applicator may be either smooth surfaced or roughened so that the developer powder is carried in the depressed portions of the patterned surface. Exemplary of this system are the techniques disclosed by H. G. Greig in U.S. Pat. No. 2,811,465.
Liquid development may also be employed in the development of charge patterns. In conventional liquid development, more commonly referred to as electrophoretic development, an insulating liquid vehicle having finely divided solid material dispersed therein contacts the imaging surface in both charged and uncharged areas. Under the influence of the electric field associated with the charged image pattern the suspended particles migrate toward the charged portions of the imaging surface separating out of the insulating liquid. The electrophoretic migration of charged particles results in the deposition of the charged particles on the imaging surface in image configuration.
An additional liquid technique for developing charge patterns is the liquid development process disclosed by R. W. Gundlach in U.S. Pat. No. 3,084,043. In this method, a charge pattern is developed or made visible by presenting to the imaging surface a liquid developer on the surface of a developer dispensing member having a plurality of raised portions defining a substantially regular patterned surface and a plurality of portions depressed below the raised portions. The depressed portions contain a liquid developer which is maintained out of contact with the imaging surface. When the raised areas of the developer applicator are brought into contact with the imaging surface bearing a charge pattern the developer creeps up the sides of raised portions in contact only with the charged area of the imaging surface, and is deposited thereon.
This technique is to be distinguished from conventional liquid development wherein there is an electrophoretic movement of charged particles suspended in a liquid carrier vehicle to the charged portion of the image bearing surface while the liquid substantially remains on the applicator surface and serves only as a carrier medium. In the liquid development method described by R. W. Gundlach in U.S. Pat. No. 3,084,043 the liquid phase actively takes part in the development of the image since the entire liquid developer is attracted to the charged portions of the image bearing surface. Furthermore in the liquid development method described by R. W. Gundlach, unlike conventional liquid development, the developer liquid contacts only the charged portions of the image bearing surface.
A further liquid development technique is that referred to as "wetting development" or selective wetting described in U.S. Pat. No. 3,285,741. In this technique, an aqueous developer uniformly and continuously contacts the entire imaging surface and due to the selected wetting and electrical properties of the developer substantially only the charged areas of the normally hydrophobic imaging surface are wetted by the developer. The developer should be relatively conductive having a resistivity generally from about 10.sup.5 to 10.sup.10 ohm. cm. and have wetting properties such that wetting angle measured when the developer is placed on the imaging surface is smaller than 90.degree. at the charged areas and greater than 90.degree. in the uncharged areas.
"Ink jets" or "ink spitters" are known to be useful for marking a recording surface with a liquid, typically in response to electrical or mechanical input which controls the trajectory of a droplet of liquid. Liquids are also applied in marking fashion to surfaces by means of a stylus which may be mechanically or electrically controlled. Such methods of marking a recording surface with a liquid are described in U.S. Pat. No. 3,573,846; 3,786,517 and 3,369,252 and in U. K. Pat. No. 1,064,344.
Although capable of marking a recording surface, the prior art systems relating to "ink jets" and to styli generally require complex systems of electrical and mechanical input to direct the ink droplets or the stylus. Additionally, the control of the "ink jets" or of the stylus must in many cases be synchronized with the movement of the recording surface.
Although such methods of developing a charge pattern with a liquid and of marking a recording surface with a liquid are capable of producing acceptable images, these methods each have several undesirable aspects which are sought to be overcome. The electrophoretic and wetting development methods of developing a charge pattern involve contact of the entire recording surface with the development liquid. Such complete contact is sometimes undesirable. The development method of R. W. Gundlach contemplates close tolerances between the developer applicator and the imaging surface. Obtaining such tolerances can be undesirably expensive and troublesome. As indicated above, the "ink jet" and stylus methods of marking with a liquid generally use undesirably complex and expensive electrical or mechanical methods of controlling the trajectory of a liquid droplet or the movement of a stylus in response, for example, to optical input.
It can be seen that a simple, inexpensive means and apparatus for marking with a liquid which overcomes the disadvantages of the prior art is desirable. Such a means and apparatus should desirably produce images of high resolution and good density on an imaging surface with a minimum of either electronic or mechanical mechanisms.
It is therefore an object of the invention to at least substantially overcome the disadvantages of the prior art.
According to one aspect of the present invention, there is provided a method of developing a charge pattern on a moving arcuate imaging surface which comprises:
a. producing a stream of electrically charged droplets of substantially regular size and spacing, the droplets having a charge polarity opposite that of the charge pattern, the stream having a path substantially tangential to the imaging surface such that the droplets pass within less than about 0.020 inch of the surface;
b. providing deflecting electrodes on opposite sides of the stream in a plane parallel to the imaging surface; and
c. applying an alternating electric voltage to the deflecting electrodes to deflect the charged droplets in a plane parallel with the imaging surface, said charged droplets having a charge sufficient to cause at least a portion of the droplets to be drawn to the imaging surface by the charge pattern thereon.
According to another aspect of the invention, there is provided an apparatus for developing a charge pattern on an arcuate imaging surface which comprises:
a. a means for producing a stream of charged droplets of substantially regular size and spacing positioned such that the stream has a trajectory substantially tangential to said imaging surface at a distance up to about 0.020 inches from the surface;
b. deflecting electrodes placed on either side of the stream of droplets in a plane parallel to the imaging surface; and
c. a means for providing a source of alternating current connected to the deflecting electrodes to spread the stream in a plane parallel with the imaging surface.
Generally, such a method provides a liquid development of high resolution and good density of a charge pattern on an imaging surface. The method of the present invention at least tends to require a simple and relatively inexpensive apparatus and to avoid complex electrical or mechanical manupulation of the liquid droplet trajectory.